1. Field of the Invention
The invention relates to the field of fabrication and implementation of a high resolution fiber optic based accelerometers.
2. Description of the Prior Art
In optics, a Fabry-Pérot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors. (Technically the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently.) Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. The resonance effect of the Fabry-Pérot interferometer is identical to that used in a dichroic filter. That is, dichroic filters are very thin sequential arrays of Fabry-Pérot interferometers, and are therefore characterized and designed using the same mathematics. Etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Pérot interferometers.
The prior art has disclosed the use of coherence division multiplexing of multiple Fabry-Perot Interferometers (FPI) based sensors in a parallel array. This scheme requires a high precision scanning reference interferometric cavity. Others have disclosed a system of six optical fiber FPI strain sensors with unique cavity path lengths which demonstrated coherence division multiplexing in series. The system required the use of a laser-referenced Michelson interferometer to resolve the signals Still others have showed a frequency domain multiplexing to resolve the signal from a serial system of three fiber FPI strain sensors. Extensive data post processing applied to data acquired from a monochromator was required to extract the sense signal. It is known to use coherence division multiplexing with three FPI temperature sensors. A temperature controlled scanning Mach-Zehnder interferometer reference is employed. It is also known to use wavelength division multiplexing of two FPI pressure and temperature sensors using finer Bragg grating or arrayed waveguide gratings.
There are no prior art systems of simply multiplexed FPI-based accelerometers in transmission. The similar systems reported in the literature above represent the present state of the art of similar sensor system technology. None of the previously proposed or developed systems use wavelength dependent reflectivity to achieve sensor multiplexing. The common characteristic of the previously developed multiplexing techniques for FPI based sensors is that these techniques require relatively complex infrastructure (optical splitters and couplers, fixed and variable optical references, gratings or waveguide systems, etc) and signal processing techniques. Sealing of these systems require multiplication of the infrastructure components. The simplicity of the system presented here is preferable for many applications.
Systems have been proposed using fiber Bragg gratings (FBG). However, since such sensors are formed in the bulk of an optical fiber, it is difficult to form sensor systems involving mobile elements (especially inertial sensors) using such systems.